This invention relates to infrared optical detection and imaging systems having a detector in a low temperature environment with an optical stop which reflects the detector upon itself to minimize interference with extraneous thermal radiation and, more particularly, to the use of a warm stop located outside the low temperature environment for limiting the field of view, and wherein the warm stop is configured as a flat reflecting plate having a Fresnel type surface profile including alternatively kinoform and binary surface profiling.
Infrared detection and imaging systems are employed in a large number of situations, such as night vision systems by way of example, wherein infrared radiation emitted by a subject is to be detected by a detector assembly having a single detector, typically a solid state detector, or a plurality of detectors for obtaining data of the subject. In the case of the single detector, the radiation may be scanned past the detector in a two dimensional scan to provide a two dimensional data field or image of the subject. In the case of a system employing the plurality of detectors, the detectors may be arranged in a line array for providing a one-dimensional line image of a line of data points of the subject, and wherein the scene of the subject may be scanned in a direction perpendicular to the line image to provide a two dimensional data field or image of the subject. Alternatively, the plurality of detectors may be arranged in a two dimensional array to provide directly a two dimensional image of the subject. The detector assembly is positioned within a housing, generally referred to as a Dewar, which maintains the array of detectors in the requisite low temperature environment. A lens assembly is usually employed to focus the subject radiation upon the detector assembly, the lens assembly being located outside the Dewar.
To prevent thermal radiation from a heat source, other than the subject, from reaching a detector and thereby degrading the image of the subject, it is a common practice to construct the Dewar with an internal optical stop set to the maximum design aperture for the system, and to provide also an optical stop outside the Dewar to limit the radiation to the field of view of the subject. The optical stop within the Dewar is referred to as a cold stop, and the optical stop outside the Dewar is referred to as a warm stop. The cold stop is cold and thus radiates only a very small amount of radiation to the cold detector. The warm stop is optimally reflective serving to reflect the image of the cold detector back upon the detector. In this fashion, the detector views only itself in addition to the radiation propagating from the subject scene through the aperture in the stop. Thereby, each stop has succeeded in excluding radiation from impinging upon the detector from any source lying outside the field of view of the subject. In order to accomplish the operation of the warm stop in reflecting the detector upon itself, it is the practice to construct the stop as a low emissivity, high reflectivity, spherical surface positioned at or near a pupil with the center of its curvature centered on or near the center of the detector or the detector array.
A problem arises in that, in the case of the reflective warm stop, the construction of the stop as a section of a sphere occupies an excessively large amount of space within the optical path of the system. This generally requires the location of the stop to be adjacent to a window of the Dewar for optimal exclusion of unwanted radiation. In contrast, the lenses of a lens assembly employed to focus the radiation may be placed at a distance from the Dewar, this allowing replacement or adjustment of the lens assembly to accommodate different imaging situations, such as the imaging of a close subject or a distant subject.
As a further problem, it is desirable also to select a warm stop to match the specific field of view of the imaging situation, such as a narrow or a wide angle field of view. However, the replacement of the warm stop is difficult because of its location immediately in front of the Dewar window. Furthermore, the optimum aperture stop shape is often not circular, this resulting in a stop that generally lies out of the pupil plane as an unavoidable consequence of the geometry involved. For example, a rectangular aperture is desirable in the situation wherein a rectangular scene is to be imaged. The calculation of the optimum geometry is time consuming and the best fit necessarily compromises the stop's efficiency. Also, it would be much more convenient to have the warm stop located within the lens assembly so that both the lenses and the warm stop could be replaced in a single step to accommodate a change in field of view or a change in magnification while preserving optimum reflective warm stop efficiency. This is precluded by the necessity to locate the warm stop at the Dewar window.